Travelers should be cautious when using public transportation systems, including maritime services like ferries, the statement said.

The Middle East and Africa are the most unpredictable areas, with terrorist groups and pirates attacking recently in Yemen, Algeria, Kenya and Somalia, the statement said.

It listed a number of places that might be targeted, including "high-profile sporting events, residential areas, business offices, hotels, clubs, restaurants, places of worship, schools, public areas, and other tourist destinations."

Maritime authorities recommend Western seafarers stay at least 200 nautical miles off Somalia and avoid the port of Mogadishu, the statement said.

The Department of State "encourages U.S. citizens living overseas or planning to travel abroad to enroll in the Smart Traveler Enrollment Program," the statement said. Officials use the program to contact enrollees in the event of an emergency. Thursday's announcement replaces one issued in January.